mojo/public: Include gtest.h as "gtest/gtest.h", instead of via "absolute" path.

mojo/public/BUILD.gn

Index: mojo/public/BUILD.gn
diff --git a/mojo/public/BUILD.gn b/mojo/public/BUILD.gn
index 1de956f7b1370a70e30da6600c488b1f34ec16f0..4c9bb3d8f8009810df881b57bc0be084ef139443 100644
--- a/mojo/public/BUILD.gn
+++ b/mojo/public/BUILD.gn
@@ -56,3 +56,13 @@ static_library("libmojo_sdk") {
     ":sdk",
   ]
 }
+
+# "Forwarding" target, to make it easy to put gtest wherever you want.
+# TODO(vtl): Maybe this should be parametrized and made into
+# //build/module_args/mojo.gni (but maybe that's overkill).
+group("gtest") {
+  testonly = true
+  public_deps = [
+    "//testing/gtest",
+  ]
+}